A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy into the ground and recording the “echoes” that return from the rock layers below. The sound energy can originate, for example, from explosions or seismic vibrators on land environments, or air guns in marine environments. During a seismic survey, the sound energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the sound energy source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at multiple locations on the surface. Multiple sound energy source and recording combinations are then combined to create a near continuous profile of the subsurface, which may extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally selected along a single line. In a three-dimensional (3-D) survey, the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey.
Also, a time lapse, often referred to as a four-dimensional (4-D) survey can be taken over the same survey area at two or more different times. The 4-D survey can measure changes in subsurface reflectivity over time. Changes in the subsurface reflectivity can be caused by, for example, the progress of a fire flood, movement of a gas/oil or oil/water contact, etc. If successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) can be attributable to the subsurface processes that actively occurring.
A seismic survey can be composed of a very large number of individual seismic recordings or traces. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2-D processing. General information pertaining to 3-D data acquisition and processing can be found in Chapter 6, pages 384-427, of Yilmaz.
A seismic trace can be a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface. A partial reflection occurs each time there is a change in the elastic properties of the subsurface materials. The digital samples in the seismic traces are often acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample, in a digital seismic trace, can be associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement can be used, e.g. vertical seismic profiles (VSP) surveys, ocean bottom surveys, etc.
Further, the surface location of every trace in a seismic survey can be tracked and made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations. The tracking allows posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
The data in a 3-D survey can be viewed in a number of different ways. First, horizontal “constant time slices” can be extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2-D plane of seismic data. By animating a series of 2-D planes, it is possible to pan through the volume, giving the impression that successive layers are being stripped away so that the information, which lies underneath, can be observed. Similarly, a vertical plane of seismic data can be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2-D seismic line from within the 3-D data volume. It should also be noted that a 3-D dataset can be thought of as being made up of a 5-D data set that has been reduced in dimensionality by stacking it into a 3-D image. The dimensions can be time (or depth “z”), “x” (e.g., North-South), “y” (e.g., East-West), source-receiver offset in the x direction, and source-receiver offset in the y direction. While the examples here can apply to the 2-D and 3-D cases, the extension of the process to four or five dimensions can be achieved.
Seismic data, which has been acquired and processed, can provide a wealth of information to an explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the processing of seismic data, estimates of subsurface rock velocities can be generated and near surface inhomogeneities can be detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Seismic waveform attributes, such as phase, peak amplitude, peak-to-trough ratio, etc., can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
Seismic data stacking is one of type of applied processing/enhancement technique for seismic data. In simplest terms, stacking can include combining multiple seismic traces into a single trace for purposes of noise reduction. Conventional stacking can be ineffective for certain types of noise (e.g., where one or a few of the traces contain high amplitude noise). As such, there have been ongoing efforts to improve the quality of seismic stacking.
Stacking can be applied in both the data and the image domain. In the discussion here we describe the method as applicable to seismic images or seismic image volumes but it could be applied to seismic data using the same algorithm. Seismic images are occasionally further decomposed into a plurality of images, each one of which correspond to a subset of attributes, for example different opening angles, vector offsets, shot directions or any other possible attribute. However, these images need to be combined in order to obtain a high quality final image stack or multiple realizations of final image stack. This has led to a renewed interest in the process of stacking. To the extent that the stacking process can be improved, the final stacked data/image quality will similarly be improved. As such, there is a need for a methods and system of producing an improved stack of seismic data beyond the simple summation of all attribute subsets.